Fluid operated actuators are generally known in the art. Fluid operated actuators convert a fluid pressure to a work piece using an actuator that typically consists of a piston in a cylinder. A piston rod coupled to the piston typically acts on the work piece to perform some task. Although there are various suitable fluids that may be used, the fluid applied to the actuator generally comprises pneumatic or hydraulic fluid, for example. One particular application for fluid operated actuators is in blow molding systems.
Blow molding is a generally known process for molding a preform part into a desired product. The preform is in the general shape of a tube with an opening at one end for the introduction of pressurized gas, typically air; however, other gases may be used. One specific type of blow molding is stretch blow molding (SBM). In a typical SBM application, a valve block provides both low and high-pressure gas to expand the preform into a mold cavity. The mold cavity comprises the outer shape of the desired product. SBM can be used in a wide variety of applications; however, one of the most widely used applications is in the production of Polyethylene terephthalate (PET) products, such as drinking bottles. Typically, the SBM process uses a low-pressure fluid supply in combination with a stretch rod that is inserted into the preform to stretch the preform in a longitudinal direction and radially outward and then uses a high-pressure fluid supply to expand the preform into the mold cavity. The low-pressure and high-pressure supply can be controlled using one or more blow-molding valves. The resulting product is generally hollow with an exterior shape conforming to the shape of the mold cavity. The gas in the preform is then exhausted through one or more exhaust valves. This process is repeated during each blow-molding cycle.
With each blow-molding cycle, a blowing cylinder (also known as a capping cylinder) extends a nozzle towards the preform in order to form a fluid-tight seal with the preform and/or the mold cavity. Because of the extremely high pressures encountered in blow molding systems, it is important that the nozzle can be positioned in an accurate and repeatable manner. Prior art blowing cylinders typically provide a relatively large piston head in order to increase the speed at which the nozzle can be lowered for a given actuating pressure. The problem with this approach is that as the piston head size increases, the system is faced with an increasingly difficult task of maintaining concentricity (centering the piston within the bore) as the piston and nozzle are lowered onto the preform. In addition, these prior art approaches can result in premature damaging of the nozzle, the preform, or both as the nozzle impacts the preform with a relatively high amount of force due to the increased speed of travel. Prior art approaches have not satisfactorily developed a way to cushion the impact as the nozzle is lowered onto the preform.
The embodiments described below overcome these and other problems and an advance in the art is achieved. The embodiments provide a multiple-stage fluid operated actuator with a multiple-piece piston assembly. In some embodiments, the fluid operated actuator comprises a blowing cylinder assembly for a blow molding system with a multiple-piece piston assembly. The multiple-piece piston assembly is capable of maintaining an increased degree of concentricity while simultaneously cushioning the impact of the nozzle as it reaches the end of travel to contact the preform and/or the blowing cavity.